Recovery of phosphorus from sludge



June 9, 1964 J. C. BARBER ETAL RECOVERY OF PHOSPHORUS FROM SLUDGEOriginal Filed June 19, 1961 PHOSPHORUS CONDENSER LUDGE MK? WATER TOCONDENSER SPRAYS 5 Sheets-Sheet 1 WZ INVENTORS.

J. c. BARBER ETAL 3,136,604

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United States Patent RECOVERY OF PHOSPHORUS FROM SLUDGE James C. Barber,George H. Megar, and Thomas S.

Sloan, Florence, Ala., assignors to Tennessee Valley Authority, acorporation of the United States Original application June 19, 1961,Ser. No. 118,204, now

Patent No. 3,084,029, dated Apr. 12, 1963. Divided and this applicationFeb. 19, 1962, Ser. No. 174,319

3 Claims. (Cl. 23165) (Granted under Title 35, US. Code (1952), see.266) This application is a division of our copending application, SerialNumber ll8,204, filed June 19, 1961, now US. Patent 3,084,029, forRecovery of Phosphorus From Sludge.

Our invention relates to an improvement in the production and recoveryof phosphorus, and more particularly to an improved process for treatingphosphorus sludge formed during the production of phosphorus by smeltingphosphate rock.

Heretofore it has been the practice in the chemical industry to produceelemental phosphorus from phosphate rock by reducing the phosphate rockwith coke or other carbonaceous reducing agents in the presence ofsilica. Such a process is normally carried out in equipment such as anelectric phosphorus smelting furnace in which the phosphorus vaportherefrom carries with it such foreign matter as particles of rock,sand, reducing agent, and fluorine compounds. The result of such processis that when the phosphorus vapor is condensed and collected under waterin order to recover the phosphorus some of the product upon removal fromthe condenser is found to be of the character of a sludge containingvarious concentrations of phosphorus as well as the above-mentionedimpurities and water.

It has been the practice to install electrostatic precipitators at thephosphorus furnaces for the purpose of removing the above-mentionedsolid impurities from the gases prior to the condensation of phosphorus.However, the precipitators have been found to remove only about 60 to 90percent of the solids in the gas. .The gases are then cooled with waterspray to condense the phosphorus vapor to liquid phosphorus. The solidimpurities that remain in the gas after passing through theelectrostatic precipitators are also collected in the water-phosphorusmixture that drains from the condenser. In the condenser sump (vesselinto which the condenser is drained) a layer of relatively high-gradephosphorus is obtained at the bottom; above this layer is a mixturereferred to as sludge which consists of phosphorus droplets or globules,solid impurities, and water; and above the second layer is a water layercontaining phosphorus droplets and solids in suspension. The boundarybetween the sludge and the water suspension is not clearly defined.

The sludge is viscous and sometimes sticky; it consolidates duringstorage and becomes more viscous. It is very difficult to pump and burnconsolidated sludge in conventional burner assemblies. Also, the acidproduced by burning the sludge is contaminated.

Various methods of preparation and recovery of the phosphorus from thesludge have been suggested in the prior art; among these are filtration,distillation, and briquetting methods.

Phosphorus in the sludge can be separated from its impurities bydistillation, but this method of separation has several disadvantageswhen practiced on a commercial scale. Phosphorus losses are high as aresult of the formation of red phosphorus which remains in the residue,and some phosphorus is lost as uncondensed vapor. Also, investment costsfor a distillation assembly are high.

Our invention is directed to an improved process for recoveringphosphorus from sludge by means utilizing the "ice introduction of smallamounts of dispersing agents into the sludge to be treated. We havediscovered that the use of as little as 1.5 parts of our dispersingagent per 1,000 parts by weight of sludge substantially reduces thestickiness of the sludge, and the viscosity of the treated sludge isreduced so as to result in a material which is sufficiently fluid as tobe readily pumped to and burned in conventional phosphorus burners torecover, as P 0 the phosphorus values from said sludge.

We have found that the use of our dispersing agent increased the pH ofthe sludge froma value of about 3.0 up to a value of about 6.0, and thatthe amount of dispersing agent used is highly critical in that excessiveuse thereof causes the pH of the slurry to rise to above 7.0 with theresulting formation of phosphine gas.

It is therefore an object of the present invention to provide animproved method of treating phosphorus sludge by a process in whichsubstantially all of the elemental or uncombined phosphorus contained inthe sludge can be removed in a simple and economical manner.

Still another object of the present invention is to provide a method oftreating the phosphorus sludge whereby a substantial part of anycombined phosphorus contained in the sludge can be recovered as P 0 Afurther object of the present invention is to provide an improved methodfor recovering phosphorus from sludge in which the sludge isconsolidated and is treated with a small but critical amount ofdispersant to reduce its viscosity and make it readily pumpable toensure ease in burning same to recover therefrom the phosphorus values.

A still further object of the present invention is to provide animproved method for recovering phosphorus from sludge in which thesludge is consolidated and is treated with a small but critical amountof dispersant to reduce its viscosity and make it readily pumpable, andin which the cost of treating the sludge with dispersant is in the rangeof about 30 cents to 60 cents per ton.

A still furtherobject of the present invention is to provide an improvedmethod for recovering phosphorus from sludge in which the sludge isconsolidated and is treated with a small but critical amount ofdispersant to reduce its viscosity and make it readily pumpable, inwhich the cost of treating the sludge with dispersant is in the range ofabout 30 cents to 60 cents per ton, and in which substantially largeramounts of low-cost reducing agents (bituminous coal) may be used in thephosphorus smelting furnaces.

Another object of the present invention is to provide an improved methodfor the recovery of phosphorus from sludge in which method phosphorusparticles in the sludge are caused to coalesce by means of appliedcentrifugal force with the resulting recovery of elemental phosphorustherefrom.

Still another object of the present invention is to provide an improvedmethod for the recovery of phosphorus from sludge in which method thephosphorus particles in the sludge are caused to coalesce ,by means ofapplied centrifugal force with the resulting recovery of elementalphosphorus therefrom, and in which the impurities removed from thesludge may be utilized as a binder to agglomerate furnace feedmaterials.

In carrying out the objects of the present invention in one formthereof, we employ a method of effectively decreasing both the viscosityand the stickiness of the sludge through the use of an extremely smallamount of effective dispersing agent selected from a group ofcommercially available lignosulfonates. In addition, we have alsodiscovered that the use of sodium hydroxide as the dispersing agentprovides results which indicate that it is almost as effective as thelignosulfonate agents, and that, furthermore, the use of such sodiumhydroxide material as a dispersing agent may prove to be somewhat moreeconomical than the use of a lignosulfonate dispersant.

Our invention, together with further objects and advantages thereof,will be better understood from consideration of the followingdescription, taken in connection with the accompanying drawings, inwhich:

FIGURE 1 is a diagrammatical illustration of a phosphorus condenser sumpshowing how the phosphorussludge and the water-suspension layers arecollected in the sump.

FIGURE 2 is a diagrammatical illustration of the preferred embodiment ofthe present arrangement for fluidizing sludge with our dispersing agent.

FIGURE 3 is a diagrammatical illustration showing an alternative methodof recovering phosphorus by means of centrifuging the sludge to causecoalescence of the phosphorus particles therein and to permit recoveryof elemental phosphorus as a high-grade material.

Referring now more specifically to FIGURE 1, there is shown a condensersump 1 in which a layer of relatively high-grade phosphorus is obtainedas bottom layer 2. Above bottom layer 2 is shown a layer of the sludge 3which consists of phosphorus droplets or globules, solid impurities, andwater. Above layer 3 is shown water layer 4 containing phosphorusdroplets and solids in suspension.

Microscopic examination of the sludge shows that it consists of globularparticles of yellow phosphorus, 1 to 2 millimeters in diameter down to afew microns in size. Fine particulate foreign matter is associated withthe phosphorus. There is little tendency for the phosphorus particles tocoalesce; however, some of the smaller globules agglomerate intoclusters. Electrical charges and physical barriers of foreign solids arethought to retard or prevent coalescence of this phosphorus.

The solids in the sludge result from the precipitation of solids in thecondensing system'and particles in the furnace charge being carried overinto the condensing system. Following is the approximate composition ofthe principal solid fractions of the sludge: 32 percent P 23 percent F,8 percent CaO, and 7 percent SiO the remaining fractions being water,carbon, and carbonaceous materials generally associated with phosphorussludges. The amounts of P 0 and F in the sludge in one system were foundto be about 2.1 and 1.6 tons per day, respectively.

When bituminous coal is used as a reducing agent in the phosphorusfurnaces, some of the carbonaceous material in the coal is volatilizedand carried over to the condenser. The carbonaceous material collects inthe sludge layer and causes the sludge to be sticky and more difficultto handle. Bituminous coal is much less costly than coke (which isnormally used as the reducing agent); however, the amount of coal thatcan be used is limited because of the adverse effect on the sludgeproperties. We have found that the amount of low-volatile bituminouscoal used in the furnaces is limited to about half of the total reducingagent requirement because of the adverse effect of the carbonaceousmaterial on the handling properties of the sludge. The development ofour method to cope with the sludge problem results in greater economiesin the smelting furnaces by allowing the use of larger amounts ofbituminous coal or the use of lower cost (high volatile-matter content)coal.

The sludge collected in condenser sump 1 may be pumped to an acid unitand burned, or the material may be pumped to storage tanks for later usein the production of phosphoric acid. When the sludge is burned shortlyafter its collection, some of the impurities enter the acid and cause itto be contaminated. This problem is of greater concern when bituminouscoal is used in the furnaces, because the carbonaceous material is notcompletely burned and carbon sometimes collects in the acid. Inaddition, fluorine is evolved when the sludge is burned,

and the fluorine causes serious corrosion problems in the acid unit.

On .the other hand, if the sludge is stored for a time before it isburned, some of the phosphorus slowly collects asa liquid layer'in thebottom of the sump, as is shown in FIGURE 1; consequently, thephosphorus content in the sludge layer decreases. Upon prolonged storagethe sludge layer consolidates and becomes extremely viscous. Thephosphorus content decreases so much that it may be diflicult orimpossible to burn the material in an acid unit. The presence ofcarbonaceous material causes the sludge to be sticky and furthercomplicates its pumping, mixing, and burning.

Referring now more specifically to FIGURE 2, sodium hydroxide solutionis fed through linell and means for control 12 into storage tank 13.Mixing of the contents of tank 13 is effected by recirculation with pump14. It has been found that the contents of tank 13 can be mixed byrecirculation with pump 14 in a convenient manner; however, mixing canalso be accomplished, perhaps more quickly, by the use of a mechanicalagitator, not shown. Internal steam coils 15 are used as a means forheating the contents in tank 13. The sodium hydroxide solutiondispersing agent is added slowly over a period of about 2 hours untilthe pH of the sludge in the tank increases to about 6.0. The sludge isthen further mixed for approximately an additional 5 or 6 days, and thenmay be stored for an additional period of time prior to burning in aphosphoric acid unit.

Referring now more specifically to FIGURE 3, there is shown analternative method of coalescing phosphorus globules in the sludge andseparating liquid phosphorus from its impurities by centrifugal means.Sludge in compartment 20 is pumped by means 21 through strainers 22 and23 through sludge meter 24, and into disk-type centrifuge 25 via line26. Hot water from water heater 27 is led through line 28, through meter29, and introduced into centrifuge 25. Underflow from centrifuge 25 isfed via line 30 into underflow tank 31 and then to storage tank 32. Theunderflow is a liquid material containing more than about percent byweight phosphorus. Overflow is fed by line 33 into overflow tank 34 andthen by line 35 into settling tank 36. The overflow is a slurrycontaining solid impurities and carbonaceous material originally presentin the sludge. The phosphorus content of the overflow material isusually less than about 3 percent by weight. The phosphorus recovered inthe underflow and collected as a layer in storage tank 32 is suitablefor use in any subsequent fertilizer process.

In order that those skilled in the art may better understand how thepresent invention can be practiced, the following examples are given byway of illustration and not by way of limitation. All percentagesindicated therein are by weight unless otherwise so stated.

EXAMPLE I Phosphorus was produced in an electric furnace from a mixtureof phosphate rock, coke, and silica. Impure phosphorus collected in thecondenser sump was pumped to storage tanks. Phosphorus settled out ofthe impure phosphorus mixture and was pumped off, leaving a consolidatedviscous sludge. The sludge in the different storage tanks containedvarious phosphorus contents and was 69 percent at the bottom, 44 percentin the middle,

and 14 percent at the top of the sludge layer. Piping was installed inthe tank to permit mixing by recirculation of the contents from one endof the tank to the other, as is shown in FIGURE 2. The mixing of thecontents of the tank by recirculation with a pump was a convenientmixing method, but perhaps the mixing could have been accomplished muchmore quickly by use'of a mechanical agitator. The contents of the tankwere heated by means of internal steam coils to a temperature of 140 F.A 25 percent sodium hydroxide solution Was added slowly (a total of 150pounds of NaOH was added over a period of about 2 hours), after whichthe pH of the sludge was found to have increased to about 6.0. Excessesof NaOH (pH values above 7.0) would cause hazardous phosphine gas toform. The sludge was mixed 72 hours by pumping from one end of the tankto the other, after which time analysis showed relatively uniformphosphorus content. After storage for 3 days, the sludge was burned in aphosphoric acid unit to produce superphosphoric acid. Data on theacid-plant operation are given in Table 1.

Table 1 OPERATING DATA FOR ACID UNIT WIIILE BURNING FLUIDIZIGD SLUDGIGDuration of test, hours 6 Sludge burning rate, pounds per hour 1 4,545Excess air, percent 18.0 Combustion chamber outlet temperature, F. 1,163Venturi scrubber inlet temperature, F. 192 Composition of acid produced,percent:

H PO 106.1 P 0.4 Suspended solids 0.2 C 0.016

F 0.010 Composition of exhaust gas, percent by volume:

CO 0.7 0 3.8 N 95.5

Equivalent to a phosphorus burning rate of 2,500 pounds per hour.

EXAMPLE II Phosphorus was produced in an electric furnace by thereduction of phosphate rock with a mixture of coke and medium-volatilebituminous coalas reducing agent, and silica as a flux. Phosphoruscollected in the condenser sump (as is shown in FIGURE 1) as a yellowliquid layer in the bottom of the sump. A sludge layer was found on topof the liquid phosphorus layer. The sludge contained carbonaceousmaterial that resulted from the use of bituminous coal as a reducingagent in the furnace; consequently, the sludge was a sticky, viscousmaterial. The sludge was pumped to the storage tank and, in the storagetank, consolidated to form a sticky, viscous, black mass which was verydiflicult to pump and which would not mix with phosphorus. Thestickiness of the sludge was measured by inserting a glass rod 2% inchesinto the material and weighing the amount of material adhering to theglass rod. Such a measurement showed that 2.8 grams of material stuck tothe rod at a temperature of 150 F. The stickiness of the sludgeprevented an accurate measurement of the viscosity in the usual manner,the sludge being somewhat more viscous than material that had nocarbonaceous content. The phosphorus content of the sludge was 34percent near the bottom of the layer and 31 percent near the top. The pHwas 2.6. A percent sodium hydroxide solution was added to the sludge,and the contents of the tank were mixed as indicated in FIGURE 2. About180 pounds of NaOH was added to about tons of sludge in the tank. Thestickiness of the sludge, as measured by the glass-rod method, was zeroafter NaOH addition and mixing. The material was sufficiently fluid tobe readily pumped and burned.

EXAMPLE III The following example follows the alternative embodielectricfurnace by the reduction of phosphate rock with a mixture of coke andlow-volatile bituminous coal. Phosphorus in the sludge was present asglobules mixed with solid particles, carbonaceous material, and water.The phosphorus content of the sludge was 38 percent. The sludge wascentrifuged in the assembly as shown in FIGURE 3. This assemblyconsisted of strainers, a sludge meter, disk-type centrifuge, hot waterheater, and necessary pipes and pumps. The sludge feed rate was 3.5gallons per minute, and hot water (190 F.) was added to the sludge at arate of 1.5 gallons per minute. The mixture of sludge and hot waterflowed through strainers having openings equivalent to 40 mesh.Additional hot water (0.5 gallon per minute) was used as wash water inthe centrifuge. The centrifuge operated so as to impart a centrifugalforce of 5,200 times gravity to the sludge-water mixture. Thecentrifugal force caused the phosphorus globules in the sludge emulsionto coalesce to liquid phosphorus having a greater density than theimpurities in the sludge, and be discharged as underflow from thecentrifuge. Underflow rate was 1.2 gallons per minute, and the overflowrate was 4.3 gallons per minute. Underflow contained 92 percentphosphorus (93 percent of the total phosphorus in the feed). Phosphoruscontent of the overflow was 2.7 percent (7 percent of the totalphosphorus in feed), Phosphorus recovered in the underflow was suitablefor use in any of the fertilizer processes. Overflow was a slurrycontaining the solid impurities and carbonaceous material originally inthe sludge. The settled solids obtained as overflow from the centrifugewere used as an experimental binder to agglomerate a mixture of calcinedphosphate, coke, and silica. (Mixture consisted of 69.8 percent minus8-mesh nodule fines, 9.2 percent minus 3-mesh plus 8-mesh coke, and 11.0percent building sand.) When 10 percent of the solids-carbonaceousmaterial mixture was added to the phosphate-coke-silica mixture andbriquetted at a pressure of 5,000 pounds per square inch, cylindricalbriquets 1% inches in diameter, which had a crushing strength of pounds,were formed in a laboratory press. After heating the briquets in anatmosphere of CO at 1050 F., the strength of the briquets was 820pounds. Thus, We have discovered that the sticky carbonaceous materialin the sludge resulting from the use of bituminous coal as a reducingagent in phosphorus furnaces maybe recovered by centrifuging, and thisimpurity may be utilized as a binder for agglomeratingphosphorus-furnace charge. Or, the impurities in the sludge may bereturned to the rawmaterial process, the carbonaceous material burnedout, and the P O value recovered as feed for the phosphorus furnaces.

EXAMPLE IV Aqueous effluent from the phosphorus condensers containingsolids and globules of phosphorus was fed to the centrifuge assemblyshown in FIGURE 3 at a rate of 7.8 gallons per minute. Wash water wasadded at a rate of 2.0 gallons per minute. centrifuging of this effluentin a manner similar to that described in Example III caused thesuspended phosphorus particles to coalesce to liquid phosphorus. Thecentrifugingforces developed by the centrifuge caused almost completecoalescence of the phosphorus particles and resulted in the recovery of99.9 percent of the phosphorus in the centrifuge feed. The recoveredphosphorus collected as underflow from the centrifuge contained 64percent phosphorus, dry basis.

EXAMPLE V Consolidated sludge was treated with a dispersing agent todecrease viscosity and stickiness according to Example I above. Thetreated sludge was then centrifuged to recover the phosphorus inaccordance with the process shown in Example III above. Treatment withthe dispersant permitted the sludge to be. more readily pumped andhandled. Such a treatment with dispersant facilitates the centrifugingoperation, and particularly facilitates the straining of the sludgeprior to centrifuging.

EXAMPLE v1 Phosphorus was produced in an electric furnace by thereduction of phosphate rock with a mixture ofcoke and medium-volatilebituminous coal as reducing. agent, and silica rock as flux. .The.resulting phosphate-sludge mixture was allowed to separate into twolayers in the usual manner. The sludge layer contained carbonaceousmaterial and upon storage ina cylindrical storage tank the sludgeconsolidated and formed a viscous, sticky material. The phosphoruscontent of the stored sludge was 34 percent. Samples of the sludge wereremoved from the storage tank and treated with one percent by weight ofammonium lignosulfonate at a temperature of 140 F. Before thetreatmentthe sludge had a viscosity of 6,300 centipoises and0.2 gram stuck to aglass stirring rod immersed 2 /2 inches into the material. Aftertreatment the viscosity of the sludge wasreduced to 1,100 centipoises,and none of the material adheredto a glass rod immersed in the material.The pH of the sludge was essentially unchanged by ammoniumlignosulfonate addition.

While we have shown and described particular embodiments of ourinvention, modifications and variations thereof will occur to thoseskilled in the art. We wish it to be understood, therefore, that theappended claims are intended to cover such modifications and variationswhich are within the true scope and spirit of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In the method of producing phosphorus by smelting phosphate rock witha reducing agent in the presence of silica and thereafter condensing,collecting, and recovering the resulting phosphorus vapor under waterand separating accumulated phosphorus-bearing sludge from the relativelypure elemental phosphorus resulting from the condensation of thephosphorus vapor, the improvement which comprises adding less than about1 part of ammonium lignosulfonate per 1,000 parts of sludge by weight tosaid sludge, said ammonium lignosulfonate addition suflicient tosubstantially reduce the viscosity of said sludge; adding material ofsuflicient causticity to said sludge to raise the pH of said sludge fromabout 3.0 to about 6.0; agitating the resulting mixture for a period ofabout 2 to 6 days; thereafter burning the mixture; and recovering theresulting phosphorus oxide vapor.

2. In the method of producing phosphorus by smelting phosphate rock witha reducing agent in the presence of silica and thereafter condensing,collecting, and recovering the resulting phosphorus vapor under waterand separating accumulated phosphorus-bearing sludge from the sludge toraise the pH of said sludge from about 3.0 to

about 6.0; agitating the resulting mixture; coalescing the phosphorusparticles in said sludge per se by application of, in. centrifugalmeans, centrifugal forces thereto; removing from said centrifugal meansas overflow a water slurry containing solids and carbonaceous materials;and

v recovering from said centrifugal means as underflow,

liquid phosphorus.

3. In the method of producing phosphorus by smelting phosphate rock witha reducing agent in the presence of silica and thereafter condensing,collecting, and recovering the resulting phosphorus vapor under waterand separating accumulated phosphorus-bearing sludge from the relativelypure elemental phosphorus resulting from the condensation of thephosphorus vapor, the improvement which comprises adding less than about1 part of ammonium lignosulfonate per 1,000 parts of sludge by weight tosaid sludge, said ammonium lignosulfonate addition sulficient tosubstantially reduce the viscosity of said sludge; adding material ofsufiicient causticity to said sludge to raise the pH of said sludge fromabout 3.0 to about 6.0; agitating the resulting mixture; coalescing thephosphorus particles in said sludge per se by application of, incentrifugal means, centrifugal forces thereto; removing from saidcentrifugal means as overflow containing less than about 7 percentphosphorus by weight, a water slurry containing solids and carbonaceousmaterials; and recovering from said centrifugal means liquid underflowcontaining more than about 93 percent phosphorus by weight.

References Cited in the file of this patent UNITED STATES PATENTS417,943 Readman Dec. 24, 1889 452,821 Wing May 26, 1891 1,334,474Waggaman Mar. 23, 1920 1,788,838 Lang Jan. 13, 1931 2,039,297 Curtis May5, 1936 2,135,486 Almond Nov. 8, 1938 2,267,077 Burke Dec. 23, 1941-2,302,956 Retailliau Nov. 24, 1942 2,744,866 Kahler May 8, 1956 OTHERREFERENCES Schwartz et al.: Surface Active Agents and Detergents, vol.2, pages -102, Interscience Publishers, Inc., New York, 1958.

1. IN THE METHOD OF PRODUCING PHOSPHOROUS BY SMELTING PHOSPHATE ROCKWITH A REDUCING AGENT IN THE PRESENCE OF SILICA AND THEREAFTERCONDENSING, COLLECTING, AND RECOVERING THE RESULTING PHOSPHORUS VAPORUNDER WATER AND SEPARATING ACCUMULATED PHOSPHORUS-BEARING SLUDGE FROMTHE RELATIVELY PURE ELEMENTAL PHOSPHORUS RESULTING FROM THE CONDENSATIONOF THE PHOSPHORUS VAPOR, THE IMPROVEMENT WHICH COMPRISES ADDING LESSTHAN BOUT 1 PART OF AMMONIUM LIGNOSULFONATE PER 1,000 PARTS OF SLUDGE BYWEIGHT TO SAID SLUDGE, SAID AMMONIUM LIGNOSULFONATE ADDITION SUFFICIENTTO SUBSTANTIALLY REDUCE THE VISCOSITY OF SAID SLUDGE; ADDING MATERIAL OFSUFFICIENT CAUSTICITY TO SAID SLUDGE TO RAISE THE PH OF SAID SLUDGE FROMABOUT 3.0 TO ABOUT 6.0; AGITATING THE RESULTING MIXTURE FOR A PERIOD OFABOUT 2 TO 6 DAYS; THEREAFTER BURNING THE MIXTURE; AND RECOVERING THERESULTING PHOSPHORUS OXIDE VAPOR.